A lithium ion secondary battery, which is one type of nonaqueous electrolyte secondary battery, is chargeable at a high energy density and is dischargeable at a high voltage. Therefore, the lithium ion secondary battery is widely used as a main power source of mobile communication devices, mobile electronic devices and the like. Recently, these devices have been desired to be more compact and to provide higher performance. Accordingly, there is a need to develop a lithium ion secondary battery having still higher performance.
Lithium ion secondary batteries practically used today mainly adopt a graphite material as a negative electrode active material. However, these lithium ion secondary batteries already have a capacity close to the theoretical capacity of the graphite material (about 370 mA/g), and so it is difficult to further raise the energy density significantly. Therefore, use of various novel materials as negative electrode active materials which can further raise the capacity of the lithium ion secondary battery has been studied. For example, metal materials capable of occluding and releasing lithium, such as silicon, tin and the like, alloys containing such metal materials and the like have been proposed as negative electrode active materials and are expected to significantly raise the battery capacity.
As positive electrode active materials, lithium transition metal composite oxides having a layered structure or a spinel structure such as lithium cobalt oxide, lithium nickel oxide, lithium manganese oxide and the like are in wide use today. It is also being studied to develop a positive electrode active material capable of realizing a higher energy density than these materials. However, no material has been found so far which is superior to these materials used today in terms of comprehensive characteristics including reversibility and stability of reaction caused by charge/discharge, electron conductivity and the like.
By contrast, layered lithium transition metal oxides such as lithium cobalt oxide, lithium nickel oxide and the like which are used today may possibly be usable to realize a lithium ion secondary battery having a still larger capacity by improving the utilization factor of lithium, which forms the crystals of these oxides. One exemplary method for improving the utilization factor of lithium is to charge the lithium ion secondary battery to a still higher voltage so as to release a larger amount of lithium from the positive electrode active material during the charging of the battery. With this method, the battery is charged by raising the potential of the positive electrode to a still higher level. As a result, the average voltage during the discharging of the battery is raised, which is expected to provide an effect that the energy density of the battery is increased in terms of both of the capacity and the voltage.
However, this method of improving the utilization factor of lithium results in a larger amount of lithium being detached from the crystalline structure. This involves the possibility that the crystalline structure becomes unstable and the positive electrode active material itself is decomposed. As a measure for avoiding this, it has been proposed to stabilize the positive electrode active material by, for example, adding a different type of chemical element or by using a plurality of transition metal materials as main chemical elements to control the crystalline structure and the number of valence of the chemical elements.
With this method, the lithium ion secondary battery needs to operate in a high potential region exceeding 4 V. This requires the active material and also the electrolyte solution to have a high level of oxidation resistance. Lithium ion secondary batteries widely used in the market today generally adopt an electrolyte solution obtained by dissolving a lithium salt in an organic solvent mainly containing a chained or cyclic carbonate-based solvent. In a high potential region, such an electrolyte solution is oxidized and deteriorated by a reaction with materials of the positive electrode which is in a charged state, for example, a positive electrode active material. For example, a carbonate-based solvent is known to cause a reaction such as transesterification or the like and thus to be deteriorated. Such deterioration lowers the reliability of the lithium ion secondary batteries.
As means for suppressing the above-described deterioration of the electrolyte solution, it has been proposed to cover a surface of particles of the active material with a material having a high level of oxidation resistance. For example, Patent Document No. 1 discloses improving the cycle characteristic by covering a surface of the active material particles with a metal fluoride to decrease the activity of the surface of the active material and thus to suppress the reaction of the active material with the electrolyte solution.
Patent Document No. 2 discloses covering a surface of the active material particles with a metal halide to allow halogen to exist also inside the particles in an inclining manner from the surface to the inside. According to Patent Document No. 2, this controls the cycle characteristic at a high temperature and suppresses a reaction of the battery in a stored state, and thus the battery can be charged at a high voltage of up to 4.5 V.
Patent Document No. 3 discloses improving reversibility of lithium ions under a high temperature and a high voltage by allowing the material of the positive electrode to contain graphite fluoride and a metal fluoride. Especially, Patent Document No. 3 discloses that it is preferable to cover at least a part of the surface of the positive electrode active material particles formed of a lithium transition metal composite oxide or to allow the active material particles to contain fluorine.
Patent Document No. 4 refers to decomposition caused by oxidation of the separator in a battery which is set to be charged at a voltage exceeding 4.2 V. Specifically, Patent Document No. 4 reports the following: in the vicinity of a surface of the positive electrode in a high potential state, the oxidized atmosphere is stronger and so the separator in physical contact with the positive electrode is oxidized and so decomposed; therefore, micro short circuiting is likely to occur especially under a high temperature, which lowers a battery characteristic such as a cycle characteristic or a high-temperature storage characteristic. Patent Document No. 4 also proposes that, as means for solving this, in an electrode assembly including a positive electrode and a negative electrode facing each other while interposing the separator therebetween, a layer of resin having a high level of oxidation resistance such as poly(vinylidene fluoride), polytetrafluoroethylene or the like is located on a surface of the separator, the surface facing the positive electrode.                Patent Document No. 1: Japanese Laid-Open Patent Publication No. 8-264183        Patent Document No. 2: Japanese Laid-Open Patent Publication No. 2000-128539        Patent Document No. 3: Japanese Laid-Open Patent Publication No. 2007-103119        Patent Document No. 4: Japanese Laid-Open Patent Publication No. 2006-286531        